Homopolymers that use as a monomer dilactide or diglycolide, i.e. intermolecular dehydration cyclic esters of hydroxy acids, homopolymers that utilize as monomers lactones of intramolecular cyclic esters and copolyers of these monomers (hereinafter, simply referred to as polyester-based polymers) are decomposed by light, heat, enzymes or the like and taken in the recirculation to nature, and thus they have been subjected to many studies as biodegradable polymer materials from the viewpoint of safeness and environmental pollution control.
It has been known that processes for manufacturing homopolymers of dilactide or diglycolide are roughly classified into two types of manufacturing methods. Namely, they are a process that includes directly subjecting a corresponding hydroxycarboxylic acid to dehydration polycondensation to produce a polymer and a process that involves once synthesizing a dehydration cyclic ester of a hydroxy acid and then subjecting the resulting ester to ring opening polymerization to manufacture a polymer.
It is difficult for the former, the direct polycondensation method, to produce a polymer with a molecular weight of 4,000 or more (“Lactic Acid” written by C. H. Halten, p. 226, Veriag Chemie, 1971) and studies on conditions in reacting operation for high degrees of polymerization have resulted in, at most, a molecular weight of about 20,000 as shown in JP 02-52930 B. For this reason, when a still further higher-molecular weight polymer is needed, the latter, the ring opening polymerization method of a cyclic ester, has been utilized.
In addition, for processes of continuously manufacturing polyester-based polymers using such lactides and lactones, continuously manufacturing processes using aromatic polyesters and lactones are disclosed in JPs 61-281124A, 61-283619A and 61-287922 A.
Any of these processes include a reaction system by means of a dynamic mixer using a reactor having there in a screw or a paddle-type agitating blade such as a kneader or an extruder and subsequently delivering the polymer inside step by step from an inlet for charging raw materials to a product outlet. They disclose that these techniques enable the completion of reaction in a short time. Mixing operation using such a dynamic mixer, however, cannot prevent a temperature rise due to heat liberation by shear in a final stage of reaction that involves a high-viscosity polymer, while reduction of the agitation speed for avoidance of the heat liberation may cause insufficient mixing in early stages. Additionally, for completing the reaction in a short time mentioned above, an operation such as increasing the reaction temperature or increasing the amount of catalyst is inevitably required, and therefore, in addition to a similar problem, it inversely affects the water resistance of the polyester-based polymer. Furthermore, in order to prevent, for example, the leakage of cyclic esters evaporated by heating or shearing heat liberation to the outside of a system, the methods need to enhance the sealability in agitating shafts, or dispose a portion having an extremely high shearing stress for boosting the sealability by the polymer itself, and so the techniques are not suitable.
Also, with methods for continuously producing polyester-based polymers from lactides, JP 5-93050 A discloses a continuous polymerization method, a so-called CSTR continuous manufacturing process, in which raw materials in a reaction are continuously fed to a plurality of agitating vessels in series to take as the reaction time a residence time of the material from an initial reaction vessel to a final reaction vessel. However, any of these relates to a reactor using a dynamic mixer and does not teach or suggest methods that solve the difficulty in uniform agitation due to increase in viscosity of reactants, which is a problem during continuous production of a high-molecular-weight polyester-based polymer from lactides or lactones, and the difficulty in heat removal.
In other words, even further addition of a method of manufacturing a polyester-based polymer from lactides as disclosed in each technique supra leads to the difficulty in agitation by a normal mixer and sometimes to the difficulty in even taking out the reaction contents in that the molecular weight of the polymer formed is increased, with the polymer viscosity increased to a very high viscosity range of 10,000 poise to hundreds of thousands of poise. In addition, even though the reaction system is agitated using a strong mixer having a devised mixing blade, the reaction contents give rise to the movement such as a laminar flow caused by the rotation of the agitating blade, which makes it difficult to uniformly perform blend in an entire system.
Additionally, accompaniment of heat liberation by ring opening polymerization of a cyclic ester renders difficult the temperature control within the reaction vessel on account of the difficulty of homogeneous agitation along with increased viscosity, thereby leading to an uncontrollable reaction, or to the generation of a temperature distribution in the polymer to deteriorate the quality due to local heating.
As described in JPs 7-26001 A and 7-149878 A, static mixers (SMs) without a dynamic mixer have started being used to solve these problems; however, a non-moving mixing element fixed through a tube has a structure for repeating the splitting, conversion and inversion of the flow, which shows a very large resistance to a fluid itself. In other words, the pressure loss in a reaction system becomes very large, thereby making difficult the designing of a reactor and a pump, etc. Also, the production capacity is lowered due to the upper limit of the discharge pressure.
In addition, the SM is not provided with a movable section of controlling mixing, or shearing force and is allowed to be optimally designed only for certain specific operational conditions. Hence, for other cases and thus most operational conditions, the SM cannot control mixing and must be operated while constantly accompanying a certain extent or more of mixing failure and heat distribution.
Furthermore, when the aperture of the SM, that is, the sectional area for the fluid to pass through is enlarged in order to reduce a high pressure loss, the mixing failure and the heat distribution become remarkably large. Mixing failure increases the time required for stabilizing the physical properties of a product to be discharged from the start of operation and lengthens the residence time necessary for reaching a target conversion of a reaction, thereby causing instability of properties of the product.
To avoid this, when the SM is sometimes used as a loop type continuous reacting apparatus, the mixing effect is increased with increasing the number of loops while a distribution of the residence time through the reactor is proportionally enlarged. As a result, the deteriorations in polymer qualities due to decomposition and discoloration caused by heat subjection for a long while are unavoidable. In particular, in the production of a copolymer of a polymer having hydroxyl groups and/or ester linkages with a cyclic ester, the molecular weight distribution is broadened and further the uniformity of the segment length in each block is decreased as the transesterification proceeds, which in turn causes adverse affects such as broadening of crystal peaks measured by DSC. Also, increase in flow volume in the loop section makes the facilities very large, which renders the cost of the facilities high and makes the reactor not practical.
On the other hand, to avoid a problem concerning mixing in early stages, a method is proposed which involves previously preparing a homogeneous solution using a solvent etc., or conducting preliminary polymerization by means of an agitation type reaction vessel having a mixer. However, mixing of raw materials for a polymer which are a solid or a high viscous liquid at a handling temperature with a cyclic ester require long-duration treatment under heating conditions, which cannot avoid a problem similar to the problem occurring in the above-mentioned loop type continuous reactor as a reaction proceeds.
In particular, polyester-based polymers produced from these cyclic esters have an excellent property in biodegradability, while they are susceptible to hydrolysis caused by acids, alkalis, or water and also have a property of being readily subjected to molecular-weight reduction by heat. For example, in GUPTAM, C, Colloid Polym. Sol. (DEU) 260 (3) 308–311, 1982, a study on thermal decomposition rates of homopolymers of dilactide by heat-up thermogravimetric analysis in air is reported which shows that molecular-weight reduction acceleratedly occurs even in a sealed reaction vessel for the case at an elevated temperature of 250° C. or higher.
In addition, homopolymers and copolymers of dilactide are susceptible to progress in discoloring when exposed to high temperature. In other words, the conventional manufacturing methods using such cyclic esters cannot attain uniform blending because of an increase in viscosity accompanying increase in the molecular weight of the polymer, thus effecting partial deterioration due to local heating leading to creation of the problem of decline in qualities. Aside from a small-scale experiment in a laboratory, large-scale industrial production requires a preferable manufacturing method.
As described above, when producing a polyester-based polymer, uniform agitation is difficult to be performed due to the reaction system becoming highly viscous, which sometimes leads to the occurrence of thermal decomposition, discoloration, etc, as well as decline in qualities of a polymer formed and decrease in production capacity on account of a high pressure loss. Also, because blending in early stages of reaction of a polymer having hydroxyl groups and/or ester linkages with a cyclic ester, i.e. raw materials, is difficult to be performed, an economically disadvantageous process has been enforced.
The present invention solves the difficulty of uniform agitation due to increase in viscosity of reactants, which is a problem during industrial production of a high-molecular-weight polyester-based polymer, the difficulty of uniform blending of a highly viscous raw material and low viscous raw material, the difficulty of heat removal, and productivity reduction on account of a high pressure loss, and provides a process of continuously producing a polyester-based polymer having excellent quality.